In many high temperature electrochemical devices, metal and/or ceramic components are often required to be hermetically sealed each other. Development of effective seals has been one of the most critical areas of study for improving the performance of such devices.
For example, solid oxide fuel cells (SOFCs) typically have metallic interconnect components and ceramic cells, commonly referred to as PEN (positive cathode-electrolyte-negative anode). Similar interconnects also exist in a wide variety of other electrochemical devices, such as oxygen generators, and, as in SOCFs, they may simultaneously provide one or more functions. For example, they may act as a separator plate, separating gasses such as the fuel in the anode side and oxidant (air) at the cathode side in a SOFC stack; they may act as an interconnect plate, electrically connecting cells in series in the stack, and/or they may act as mechanical support, supporting the cells and stack for structure integrity.
Interconnect components have been made of heat resistant alloys, including Ni—/Fe-base superalloys, Cr-base alloys and stainless steels. When considering the thermal expansion match with ceramic cells or PEN, ferritic stainless compositions appear to offer the best choice, at least in the rigid seal design. In a SOFC stack, these interconnect components have to be joined and hermetically sealed to ceramic electrolyte YSZ on the components have to be joined and hermetically sealed to ceramic electrolyte YSZ on the PEN, and to another piece of interconnect. For electrochemical devices such as SOFC stacks to perform well, the metal-metal and metal-ceramic seals have to demonstrate good chemical, mechanical and thermomechanical stability. It has been found however, that due to weak bonding between glass and metal or between the metal and the oxide scale that forms on many heat resistant alloys, particularly the conventional ferritic stainless steels, it is often difficult to establish an effective bond and seal between a metal interconnect and a YSZ electrolyte. These difficulties also exist between metal interconnects, as between a sealing glass or braze in SOFC environments.
Depending on the alloy composition, either chromia (if Cr2O3 is the major component) or alumina (if Al2O3 is the major component) scales are typically formed on the alloy surface. An alumina scale usually provides better protection of matrix alloy than chromia scale. Due to its electrically insulating nature however, the alumina forming alloys can only be used in the interconnect component where the alumina scale can be bypassed by electrical current. Otherwise the pieces or parts of interconnect have to be made from chromia forming heat resistant alloys. The chromia formed on the alloy surface may not provide satisfactory resistance to oxidation however, particularly for corrosion resistance at a temperature range of 700-850° C., typical in many electrochemical devices. Making the problem worse, the chromia is volatile at this temperature range, and can poison the interface of electrochemical devices, leading to an increased polarization. Furthermore, the chromia scale may also create problems in device operation as a result of its chemical stability. Chromia scale can react with barium-calcium-aluminosilicate based sealing glasses, used in SOFC stacks, to form a high thermal expansion mismatch barium chromate compound, resulting in further performance deterioration of metal-metal and metal-ceramic seals.
Based on the problems and issues with interconnects and joins discussed above there remains a need for improved interconnects and improved methods for interconnecting components.